Spong, in U.S. Pat. No. 4,097,895, incorporated herein by reference, disclosed a bilayer optical recording medium which comprises a light reflective layer coated with a light absorbing layer. Bell, in U.S. Pat. No. 4,216,501, incorporated herein by reference, disclosed a trilayer optical recording medium having a transparent spacer layer interposed between the reflective and absorbing layers of Spong. Information is recorded in either medium by locally changing its optical properties; for example, by melting, ablating or otherwise changing the optical properties of the absorbing layer. The resulting change in the transmissivity or reflectivity of the recording medium in the exposed portions is detected for readout of the recorded information.
Such recording media are substantially uniform in their structural and optical properties prior to exposure and thus contain no means by which a track can be defined or followed prior to the recording of information. A recording medium having a pregrooved substrate contains such means but does not have the flexibility for changes of the track arrangement or identification after manufacturing. An alternative approach in which a portion of the light absorbing layer is removed by a laser beam thereby forming guard bands between light absorbing tracks has this flexibility, but the tracks may be damaged by the formation of the guard bands and the guard bands cannot be recorded on at a later date. It would be desirable to have a recording medium having guard bands in which the light absorbing tracks are undisturbed and in which information can be recorded at a later date.